Microwave determination related to a material such as chloride found in a cement based composition

ABSTRACT

Determinations are made related to the presence of a predetermined material in concrete under test using previously obtained model information. In one embodiment, the predetermined material includes a chloride material and the model information is obtained using a number of cured cement specimens. The model information is stored in memory, such as in the form of a look-up table. When the concrete is being inspected, one or more magnitudes of reflections coefficients are measured and such is utilized with the model information to make determinations related to the presence of the predetermined material. In developing the model information, each of the plurality of cured cement specimens is located in a bath containing the predetermined material. The bath may be pressurized. The cured cement specimens are maintained in the bath for different, known time intervals. After the known time interval for a particular specimen, it is dried and one or more magnitudes of reflection coefficients are measured. This data is utilized in preparing the model information.

[0001] The present application relates to and claims priority from priorU.S. Provisional Patent Application No. 60/219,461 filed Jul. 18, 2000,which is incorporated herein by reference.

[0002] The invention described herein was made in the performance ofwork under a National Science Foundation grant identified by#CMS-9817695.

FIELD OF THE INVENTION

[0003] This relates to the determination of the content of one or morematerials in a cement containing composition, such as the amount ofchloride in a cement based composition.

BACKGROUND OF THE INVENTION

[0004] The construction industry is interested in new techniques fornondestructive inspection of materials. Currently the techniques usedare adequate in some cases, but may not be the case in others. Onesolution to this problem would be to combine one technique with others.

[0005] Chloride has been found to corrode steel members in reinforcedstructures. In solving this problem, early detection and closemonitoring of chloride contaminated structures is essential inmaintaining these structures. A nondestructive technique for determiningchloride contamination would be beneficial.

[0006] Research has found that non-conducting materials, i.e. dielectricmaterials, can be analyzed by using microwave nondestructive testing(NDT) techniques. Microwave NDT techniques have been used to findsurface and sub-surface degeneration in layered materials due to impactdamage, and to measure the thickness of dielectric sheets. Suchtechniques also find unfilled spaces and air bubbles (locally anddistributed) in dielectric materials, and are used to locate andevaluate disbond and delamination in multi-layered structures. Microwavesignals can be used to measure the dielectric properties of a material.By knowing the dielectric properties of cement, aggregate, and sand,microwave signals can be used to measure the properties of the combinedmixture. Also, this can be used to determine the curing rate and thepresence of chemical reactions in the mixture.

[0007] Research has been conducted in this area. It has been foundwithin recent years that this technique can be utilized for inspectingcement based construction composition. Near and far field techniqueswere the two main groups studied. The near and far field regions arebased on the distance in which the sensor and the composition areseparated from each other. Ground penetrating radars are an example of afar field technique and have been used successfully. Although it hasbeen a success, there are still disadvantages to this technique. Forexample, repetitious calibration of the measurement equipment, thesystem spatial resolution, the inaccuracy of the data needed due tounwanted objects and the tedious signal processing needed to analyze thedata are drawbacks. These are avoided when using the near fieldtechnique. The setback of operating in this region is that the electricand magnetic fields are very complicated to model.

[0008] The measured magnitude of reflection coefficient is shown toincrease as a function of decreasing w/c ratio for cured cement paste.At first glance this seems inconsistent with the fact that higher watercontent should render a higher magnitude of reflection coefficientmeasured at a waveguide aperture. However, a closer look reveals thatduring the curing process water molecules bond with cement molecules,and some of the remaining free water evaporates. Thus, the water becomesless free and more bound over the curing time. Free water has muchhigher dielectric properties compared to those of cement powder, whereasbound water has similar dielectric properties to those of cement powder.In addition, higher w/c ratio specimens lose more of their free water toevaporation. Thus, the measured magnitude of reflection coefficient ofthese specimens decreases as a function of increasing w/c ratio.

[0009] The magnitude of reflection coefficient has been shown to bedistinctly correlated to the w/c ratio of cement paste, and subsequentlyto its 28-day compressive strength (moist cured for 3 days in ahydration and thereafter in an air room temperature).

[0010] A simple expression predicting the microwave reflectionproperties of cement paste as a function of time has been obtained.Consequently, the w/c ratio of a cement paste specimen may be obtainedby comparing two reflection coefficient measurements conducted severalhours or a few days apart after the paste has been cast. In addition, itis possible to correlate the compressive strength of cement paste duringcuring to the measured microwave reflection properties (as a percentageof the 28-day strength).

[0011] A relationship between the standard deviation of the magnitude ofreflection coefficient at higher frequencies and the s/c (sand/cement)ratio of a mortar specimen, has been established. Information on the w/cratio of mortar specimens is obtained when the average value of themeasurements is taken at relatively low microwave frequencies.

[0012] Mortar is a homogeneous dielectric mixture (even when measured ata frequency of 10 GHz). A simple dielectric mixing model has beenobtained which predicts the constituent volume content of a mortarspecimen. Consequently, the porosity (volume content of distributed air)of a mortar specimen can also be determined.

[0013] The statistical behavior of the microwave reflection propertiesof concrete as a function of w/c, s/c and ca/c (coarse aggregate/cement)ratios and the frequency of operation has been studied. It has beendetermined that the probability distribution functions of the measuredmagnitude of reflection coefficient of concrete, measured at high andlow frequency bands, possess distinct and well-known distributions. Athigher frequencies, the distribution is Gaussian whereas at lowfrequencies the distribution is uniform. With the use of the modifiableparameters in each of these distributions, the constituent volumedistribution of a given concrete mixture can be determined from itsscattering characteristics.

[0014] Similar to mortar, the results of the reflection propertymeasurements indicate that the w/c ratio in concrete, and hence itsstrength, can be correlated to the average value of the magnitude ofreflection coefficient measured at several independent locations on aspecimen at lower frequencies (i.e., about 3 GHz). At lower frequenciesthe influence of aggregate size distribution is less on the measuredmagnitude of reflection coefficient than at higher frequencies since theaggregates electrically “look smaller” at lower frequencies.

[0015] Similarly, the standard deviation and the statisticaldistribution of the measured magnitude of reflection coefficient athigher frequencies is a function of the aggregate size and volumedistributions. Hence, the constituent volume fraction and distributionof a concrete specimen may be determined at higher frequencies (i.e.,about 10 GHz).

[0016] It has been shown that the cure state of concrete specimens,containing different w/c ratios and constitute makeup, can beunambiguously determined when making daily measurements of the magnitudeof reflection coefficient.

[0017] It has also been shown that the w/c ratio of fresh concrete canbe unambiguously determined independent of its s/c and ca/c ratios. Thisis an important finding since now an operator is capable of determiningthe w/c ratio of a batch plant concrete at the time of pouring.

[0018] It has been demonstrated that the extent of aggregate segregationin concrete placement can be evaluated using the statistics of themeasured magnitude of reflection coefficient. This information can beeasily obtained for concrete members such as walls and columns in whichaggregate segregation may be an important practical issue.

[0019] Using an optimal frequency of operation, it has been effectivelydemonstrated that using a simple near-field and nondestructive microwaveinspection technique employing an open-ended rectangular waveguide probeat 3 GHz (S-band) one can easily distinguish between empty andgrout-filled masonry cells. In addition, a simple and extremelyeffective custom-built microwave inspection system has been designed andassembled for this purpose. This system has been successfully tested ona variety of masonry blocks.

[0020] Up to this point, the near field microwave NDT technique has beensuccessfully applied to the inspection and characterization of cementbased materials in several studies including detection of rebar inreinforced concrete; determination of variations in aggregate sizedistribution in concrete; determination of compressive strength andwater-to-cement (w/c) ratio of hardened cement paste (cement and water);prediction of the microwave reflection properties of mortar (cement,sand and water) using a dielectric mixing model as a tool for obtainingthe volume fraction of individual constituents of mortar; determinationof the distributed porosity in mortar; determination of sand-to-cementratios in mortar using the stochastic properties of its microwavereflection properties; and determination of the coarse aggregatevolumetric distribution in concrete.

[0021] Concrete normally provides reinforcing steel with adequatecorrosion protection. When steel is encased in concrete, a protectiveiron oxide film forms at the steel-concrete interface due to the high pHlevel associated with concrete. This film protects the steel fromcorrosion. However, the intrusion of chloride ions in reinforcedconcrete can destroy this protective film. If moisture and oxygen arepresent in the concrete, the steel will corrode through anelectrochemical process. Once the steel begins to corrode, the concretewill deteriorate. This occurs because the byproducts of corrosion occupya greater volume than the steel itself, which exerts a substantialstress on the surrounding concrete.

SUMMARY OF THE INVENTION

[0022] In accordance with the present invention, a determination is maderelated to the presence of at least one predetermined material inconcrete or cement sample. In one embodiment, the material is one thatis not normally included when the concrete is formed. For example, thematerial can be a salt that may include chloride. The salt may penetratethe concrete after it is formed. Alternatively or additionally, at leastparts of the salt might have been included with the concrete when it wasmade. In one embodiment, the presence of the predetermined material isdetected. Additionally or alternatively, the amount of the predeterminedmaterial is determined. Additionally or alternatively, a magnitude isdetermined related to the penetration of the predetermined material inthe concrete, particularly the depth that the material might be foundfrom the surface of the concrete.

[0023] An apparatus that can be used to make one or more suchdeterminations includes a signal generating subsystem, a couplersubsystem and an analyzer subsystem. With regard to making one or moresuch determinations, the signal generating subsystem outputs microwavesignals that are applied to the coupler subsystem. The coupler subsystemincludes a transmitting section that carries the microwave signals tothe concrete that is under observation or test. Reflected or returnedmicrowave signals are generated due to the incidence of the transmittedmicrowave signals on the concrete sample. These are received by thereceiving section of the coupler subsystem. These returned microwavesignals are input to the analyzer subsystem, which makes thedeterminations related to the presence, amount and/or penetrationassociated with the predetermined material.

[0024] The analyzer subsystem includes at least one memory. The memorystores model information related to the predetermined material. Inparticular, the model information includes data or other informationrelated to the predetermined material and one or more magnitudes ofreflection coefficients. These are obtainable from the reflectedmicrowave signals. They are useful in making the determinations relatedto the predetermined material. The model information is obtained basedon measurements made using cement samples that were previously analyzedunder known conditions. The model information that is obtained based onsuch testing and measurements can be presented in many different orrelated forms, such as an equation, a graph and/or a look-up table. Themodel information correlates the predetermined material in the concreteand associated dielectric property information (e.g., reflectioncoefficient magnitudes). Thus, when making determinations related to thepredetermined material for the concrete under test or in the particularcement sample, the one or more reflection coefficient magnitudesmeasured using the cement sample are found and these determinedmagnitudes are used to make determinations related to the predeterminedmaterial, such as by use of a look-up table that correlates thedetermined one or more reflections coefficient magnitudes with formationrelated to the predetermined material of interest.

[0025] With respect to obtaining the data or other information to whichthe reflection coefficient magnitudes are to be correlated, certainsteps are conducted associated with making measurements to provide suchinformation. More specifically, a cured cement specimen is made orotherwise provided. The cured cement specimen may include somepredetermined material or it may not. The cured cement specimen islocated in a bath associated with known conditions. In one embodiment,the bath is a salt bath that has chloride as the predetermined material.The cured cement specimen is maintained in the salt bath for a desiredor known time interval. The cement specimen is removed from the bathafter the known time interval. It is allowed to dry. Then, one or moremagnitudes of reflection coefficients are measured for this cementspecimen. Later at different time intervals, one or more additionalmagnitudes of reflection coefficients are determined. This is continueduntil there is essentially no change in the measured magnitudes ofreflection coefficients or such measurements are within an acceptablevariation of each other.

[0026] Additional cured cement specimens are provided. For each of thecement specimens, the foregoing steps are implemented. For at least someof these cement specimens, they are placed in the bath having thepredetermined material for different, known time intervals. Accordingly,measurements of magnitudes of reflection coefficients for other cementspecimens are made after different time intervals related to how longthe particular cement specimen remained in the bath.

[0027] In conjunction with obtaining model information related to thepredetermined material in a particular cement sample, it may bedesirable to further analyze the cement specimens after the one or moremeasurements of the magnitudes of reflection coefficients. In such acase, a cement specimen may have one or more sections removed therefrom.In one embodiment, a cylindrical cored section is removed from which anumber of smaller in height cylindrical sections (slices) are severed.Subsequently, the cored portions are ground. The ground portions aresubject to an analysis step involving an instrument, such as an electronmicroscope or the x-ray fluorescent machine, which can provideinformation related to the content of the predetermined material in thecement specimen. Such analysis can verify the accuracy of the measuringstep, as well as provide information related to penetration of thepredetermined material within the body of the concrete specimen from itssurface.

[0028] Based on the foregoing summary, a number of advantages of thepresent are readily discerned. Information related to the presence,amount and/or penetration of a predetermined material in concrete can beobtained using model information. The model information can includechloride model information. The present invention is useful when thepredetermined material is included with and/or becomes part of theconcrete after it has been formed. Substantial and extensive testing isconducted to obtain the model information, particularly using a numberof cement samples that have been cured and are subject to a bath havingthe predetermined material. Utilizing model information, such aschloride model information, the salt or chloride content of concrete canbe monitored over time related to ascertaining currently existingproperties of the concrete, such as whether its structural integrity isjeopardized by unacceptable levels of salt content.

[0029] Additional advantages of the present invention will becomereadily apparent from the following discussion, particularly when takentogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a block diagram of the apparatus of the presentinvention;

[0031]FIG. 2 diagrammatically illustrates a cured concrete specimenlocated in a pressure tank that includes, in one embodiment, a chloridesolution;

[0032]FIG. 3 is a graph illustrating magnitudes of reflectioncoefficients for cement specimens measured at S-band (3 GHz) havingknown amounts of sodium chloride;

[0033]FIG. 4 is a graph illustrating magnitudes of reflectioncoefficients for cement specimens measured at X-band (10 GHz) havingknown amounts of sodium chloride;

[0034]FIG. 5 is a graph illustrating magnitudes of reflectioncoefficients for cement specimens measured at S-band having less amountsof sodium chloride than FIG. 3;

[0035]FIG. 6 is a graph illustrating curve fits of magnitudes ofreflection coefficients for cement specimens measured at S-band for theamounts of sodium chloride of FIG. 5;

[0036]FIG. 7 is a graph illustrating magnitudes of reflectionscoefficients for cement specimens measured at X-band having less amountsof sodium chloride than FIG. 4;

[0037]FIG. 8 is a graph illustrating curve fits of magnitudes ofreflection coefficients for cement specimens measured at X-band for theamounts of sodium chloride of FIG. 7;

[0038]FIG. 9 is a graph illustrating magnitudes of all reflectioncoefficients for the cement specimens measured at S-band;

[0039]FIG. 10 is a graph illustrating curve fits of magnitudes of allreflection coefficients for the cement specimens of FIG. 9;

[0040]FIG. 11 is a graph illustrating magnitudes of all reflectioncoefficients for cement specimens measured at S-band for a different w/cratio (0.60);

[0041]FIG. 12 is a graph illustrating curve fits for magnitudes ofreflection coefficients for the cement specimens of FIG. 11;

[0042]FIG. 13 is a graph of concrete compressive strength vs. amounts ofsodium chloride for a w/c ratio of 0.50;

[0043]FIG. 14 is a graph of concrete compressive strength vs. amounts ofsodium chloride for a w/c ratio of 0.60;

[0044]FIG. 15 illustrates concrete strength curves due to the presenceof sodium chloride with a w/c ratio of 0.50;

[0045]FIG. 16 illustrates concrete strength curves due to the presenceof sodium chloride with a w/c ratio of 0.60;

[0046]FIG. 17 is a graph of magnitudes of reflection coefficients as afunction of the amount of sodium chloride after the cured specimen hasbeen in a sodium chloride bath for 28 days and where the w/c ratio is0.50;

[0047]FIG. 18 is a graph illustrating the magnitudes of reflectioncoefficients as a function of the amount of sodium chloride at day 28when the w/c ratio is 0.60;

[0048]FIG. 19 is a graph illustrating concrete compressive strength as afunction of magnitudes of reflection coefficients for certain amounts ofsodium chloride at day 28 when the w/c ratio is 0.50;

[0049]FIG. 20 is a graph illustrating concrete compression strength as afunction of the magnitudes of reflection coefficients for certainamounts of sodium chloride at day 28 when the w/c ratio is 0.60;

[0050]FIG. 21 is a graph illustrating the relative permittivity of wateras a function of frequency for different amounts of salinity;

[0051]FIG. 22 is a graph illustrating the loss factor of water as afunction of frequency for different amounts of salinity;

[0052]FIG. 23 diagrammatically illustrates a multi-layered dielectriccomposite structure under inspection using a coupler subsystem (e.g., anopen-ended rectangular wave guide);

[0053]FIG. 24 diagrammatically illustrates a concrete specimen undertest using the coupler subsystem related to penetration of thepredetermined material, such as chloride, into the specimen;

[0054]FIG. 25 is a graph illustrating magnitudes of reflectioncoefficients measured over time when calcium chloride is present;

[0055]FIG. 26 is a graph illustrating dielectric properties measuredover time when calcium chloride is present;

[0056]FIG. 27 diagrammatically illustrates a number of steps related toanalyzing a cored concrete section;

[0057]FIG. 28 is a bar graph illustrating permittivity for a number ofcored slices for a concrete specimen;

[0058]FIG. 29 is a bar graph illustrating loss factor for number ofcored slices for a concrete specimen;

[0059]FIG. 30 is a graph illustrating magnitudes of reflectioncoefficients measured over a longer period of time for concretespecimens when sodium chloride is present;

[0060]FIG. 31 is a graph illustrating magnitudes of reflectioncoefficients measured over time for tap water and salt water;

[0061]FIG. 32 is a graph of loss factor measured over time for tap waterand salt water;

[0062]FIG. 33 is a graph illustrating permittivity as a function offrequency for tap water and salt water at day 9; and

[0063]FIG. 34 is a graph illustrating loss factor as a function offrequency for tap water and salt water at day 9.

DETAILED DESCRIPTION

[0064] With reference to FIG. 1, one embodiment of an apparatus isillustrated for making determinations and obtaining information relatedto a predetermined material in concrete C under test. This embodimentincludes a signal generating subsystem 100 that outputs microwavesignals at desired frequencies and which are to come in contact with theconcrete under test. The signal generating subsystem 100 communicateswith a coupler subsystem 104. The coupler subsystem 104 receives themicrowave signals from the signal generating subsystem 100 andcontrollably directs them to the concrete. Return or reflected signalsare returned from the concrete under test to the coupler subsystem 104and are applied to an analyzing subsystem 110. The analyzing subsystem110 is involved with making determinations related to the presence,amount, and/or penetration of the predetermined material that may befound in the concrete. In one embodiment, the predetermined material isa salt, such as a chloride material.

[0065] The coupler subsystem 104 includes an oscillator 124 forgenerating microwave signals at any one of a number of selected anddesired microwave frequencies. The output of the oscillator 124 is inputto an isolator 128 that receives the inputted microwave signals andisolates the signals from any unwanted signals entering the oscillator124. The coupler subsystem 104 has a microwave transmitting section 136and a microwave receiving section 140. The transmitting section 136 isconnected to the isolator 128 by means of a first adaptor or flange 144.The transmitting section 136 is preferably in contact with, but may bespaced from, the concrete sample C. The transmitting section 136 directsthe transmitted microwave signals relative to the concrete sample. Thetransmitted microwave signals are incident upon the concrete sample,with some of the microwave energy being absorbed. Reflected microwavesignals are generated due to the incidence of the transmitted microwavesignals upon the concrete sample and the reflected microwave signals arereceived by the receiving section 140 of the coupler subsystem 104. Thereceiving section 140 is part of a receiver assembly 148 that receivesthe reflected microwave signals in order to measure the signals inconnection with the determining a reflection coefficient magnitude (I)associated with the concrete sample. That is, a reflection coefficientmagnitude (r) is determined with reference to the reflection planedefined at the concrete sample edge that is contacted by thetransmitting section 136. In making the measurement, the receivingsection 140 of the coupler subsystem 104 communicates with a crystaldetector 152 of the receiver assembly 148. The receiving section 140 isconnected to the crystal detector 152 by means of a second adaptor orflange 156. The crystal detector 152 generates a signal as a function ofthe reflected and received microwave signals. The signal is applied to aprocessing unit 160 of the analyzing subsystem 110 for use inautomatically analyzing the reflected microwave signals in order todetermine information related to the predetermined material such asvalues related to chloride in the sample of concrete C. This will bediscussed in more detail later herein. The processing unit 160communicates with storage memory 164 for storing program code(software), together with information or data useful in correlating orotherwise using previously determined model information with themeasured reflected microwave signals. As can be appreciated, the storagememory 164 can be one or more discrete memory devices or components. Adisplay unit 168 can also be provided so that the user can immediatelysee information related to the analysis that is conducted relative tothe predetermined material that might be in the concrete sample C.

[0066] Substantial portions of the following description will be in thecontext of the predetermined material including a chloride material,although it should be appreciated that other predetermined materials, asit relates to determining their presence, amount and/or penetration inconcrete can be utilized with the present invention.

[0067] In order to detect and evaluate chloride penetration in cementbased materials (in particular concrete) one must have access tospecimens in which chloride ions have penetrated (including successivepenetration simulating real-life exposure of concrete to chloride).Furthermore, these specimens must contain various amounts of chloride sothat minimum chloride content detection can be established for thisparticular near-field microwave technique. Consequently, the followingprocedure is followed:

[0068] Referring to FIG. 2, subject the specimens mentioned above (afterthey have been cured for 28 days and their microwave reflectionproperties have been measured daily) to a chloride solution in anenclosed tank under pressure and controlled temperature. The chloridecontent in the solution, the pressure level and the amount of timerequired will be varied. The pressure level and the length of timeduring which the pressure is applied will primarily influence the depthto which the chloride penetrates into the specimens, while the chloridecontent affects the amount of present chloride. This provides a widerange of results (i.e. the amount of chloride present in the samples andthe depth to which they have penetrated). This procedure is conductedseveral successive times. At each iteration, the microwave reflectionproperties of the specimens are measured, to replicate chloridepenetration over several successive contaminations. To ensure that thechloride solution replaces the air voids in the specimens, a section ofthe tank bottom is removed and the specimen will be placed (using arubber seal) in such a way that the air voids can escape from it whenthe pressure is applied, as shown below. The amount of air that willescape is a function of the w/c ratio, s/c ratio and ca/c ratio of thespecimens.

[0069] It is imperative that the overall content of chloride that hasbeen added to a specimen be known each time the specimen is subjected tothe chloride solution. Therefore, the specimens are weighed, using asensitive scale, before and after being subjected to the chloridesolution under pressure. Furthermore, the specimens will be weighedevery day, throughout the period during which their microwave reflectionproperties are being measured (after removal from the tank). In this waythe amount of water that evaporates during the first few days, after thespecimens have been removed from the chloride solution, will be known aswell. The information regarding the amount of chloride in the specimen,the amount of water and its subsequent evaporated amount, will beincorporated in the analysis of the overall results when the microwavereflection properties are being analyzed. The soaking period should notpermit re-hydration.

[0070] Chlorides can be introduced into concrete in many ways. Tney maybe introduced into the concrete mix by the aggregates, cement,admixtures and/or the mixing water. Moreover, chlorides may enter into aconcrete structure, while in use, through exposure to deicing salts,seawater or salt air environment. Since chlorides can be introducedthrough many mechanisms, placing limits on any one of concreteconstituents (e.g. mixing water) may not always limit the total amountof chloride in the concrete. Therefore, it is important to be able tomeasure the chloride content of concrete in order to indicate thelikelihood of corrosion of its embedded reinforcing steel bars.

[0071] The limit placed on the amount of chloride content in concrete isa function of the type of structure and the environment to which it isexposed during usage. Limits on chloride content in reinforced concreteare set in two ways: water-soluble chloride ion content, and the totalchloride ion content. The two values are not substantially differentfrom one another because the water-soluble chlorides are only a part ofthe total chloride content, namely, the free chloride in the water.

[0072] Steel in concrete begins to corrode when the water solublechloride content in the concrete is about 0.15% of the cement weight. Ofthe total chloride ion content in concrete, only about 50% to 80% iswater soluble, the rest becomes chemically bound in the chemical processinvolving cement. Limits on the water-soluble chloride content inconcrete have been set by the American Concrete Institute. These limits,as a percentage (of the cement weight) are 0.06% for prestressedconcrete, 0.15% for reinforced concrete exposed to chloride in service,1.00% for reinforced concrete that remains dry or is protected frommoisture in service, and 0.30% for other reinforced concrete structures.The water-soluble chloride ion content of hardened concrete may bedetermined by a procedure similar to that reported in a Federal HighwayAdministration (FHWA) Report by Clear and Harrigan. This procedure isdestructive in nature and is performed in a laboratory on concrete thathas hardened for 2 to 42 days.

[0073] The standard approaches for determining the level of chloridecontent in concrete (i.e. the total amount of chlorides) are theprocedures outlined by the American Society for Testing and Materials(ASTM) and the American Association of State Highway and TransportationOfficials (AASHTO). Both of these standards require that a core beextracted from the structure under inspection, which is ground into apowder, and subsequently tested in a laboratory to obtain its chloridecontent. Limits on the total chloride content in reinforced concretehave been set at 0.40% of the cement weight.

[0074] All of the methods mentioned for determining chloride content inconcrete require obtaining a sample of the hardened concrete.Subsequently, it is ground up and tested following the proceduresoutlined in references. These methods present several distinctdisadvantage such as being destructive, time consuming and prohibitiveto the large scale testing of large structures. Moreover, thesestructures cannot be tested again at the same location for determiningthe progression of chloride penetration. Consequently, nondestructivesolutions are highly advantageous for chloride detection.

[0075] As mentioned earlier, microwave NDT techniques have alreadydemonstrated the ability to determine the w/c ratio in cement paste andmortar specimens. The success in doing this is mainly due to theinherent sensitivity of microwaves to the presence of bound or freewater in these materials. In the case where the chloride content in aspecimen is above a certain limit, the chloride will interact with anyfree (or bound) water and significantly affect its dielectricproperties. It has already been shown that for a given microwavefrequency range, an increase in the salinity of water significantlyincreases its microwave absorption characteristics. Based on this simplebut sound principle, the detection and evaluation of chloride content incement paste and mortar is quite feasible.

[0076] A near-field microwave nondestructive evaluation technique fordetection and evaluation of chloride content in cement paste and mortarspecimens is provided. This method uses the magnitude of reflectioncoefficient measurements measured at the aperture of an open-endedrectangular waveguide probe in S-band (2.6 GMz-3.95 GHz) and X-band (8.2GHz-124 GHz).

[0077] The process of determining the content and curing effects ofchloride on cement based compositions starts by making a set of 8″ by 8″by 8″ cement mortar cubic specimens. The first sets of mortar specimenswere made with a water-to-cement ratio (w/c) of 0.50 and asand-to-cement ratio (s/c) of 1.5. The second sets of mortar specimenswere made with a w/c of 0.60 and a s/c of 1.5. Each specimen producedhad a different amount of salt added to the water. Sets of seven sampleswere made for the w/c ratio of 0.50, Table 1 and 2 show the mixtures ofthe samples, and Table 3 and 4 shows the mixtures of the five samplesmade for w/c ratio of 0.60. Due to the number of specimens needed andonly four cubic forms provided, two sets of data were obtained for eachcase. Along with making the cubic forms, four 4″×8″ cylinders (of eachsalinity) were also made.

[0078] In Table 1, specimens 1-4 provided the first sets of data andspecimens 5-7 provided the second set. In Table 3, specimens 3-5provided the first set of data and specimens 1-2 provided the secondset. Tables 2 and 4 set out weight amounts for the specimens of Tables 1and 3, respectively. Each of these samples were moist cured in ahydration room for one day and air cured for the remaining 28 days.Every day the reflection properties of the specimens were monitored atS- and X-band using an apparatus of FIG. 1, which preferably included aHP8510 vector network analyzer. Twenty points were drawn on the sides ofthe blocks and measured at these same locations every day. After the 28days of curing, the cylinders were tested for strength value with acompression machine. The data collected from both the network analyzerand compression machine were analyzed and such is found in Tables 5-7.TABLE 1 Ratio of the individual constituents for the mortar specimens.w/c ratio s/c ratio NaCl/c (%) Specimen (by weight) (by volume) (byweight) 1 0.50 1.5  0.00 2 0.50 1.5 0.1 3 0.50 1.5 0.2 4 0.50 1.5 0.3 50.50 1.5 1.0 6 0.50 1.5 2.0 7 0.50 1.5 3.0

[0079] TABLE 2 Weight of the individual constituents for the mortarspecimens. Specimen Cement (lb.) Water (lb.) Sand (lb.) Salt (lb.) 125.256 12.628 31.751 0.000 2 25.256 12.628 31.751 0.025 3 25.256 12.62831.751 0.051 4 25.256 12.628 31.751 0.076 5 25.256 12.628 31.751 0.253 625.256 12.628 31.751 0.505 7 25.256 12.628 31.751 0.760

[0080] TABLE 3 Ratio of the individual constituents for the mortarspecimens. w/c ratio s/c ratio NaCl/c (%) Specimen (by weight) (byvolume) (by weight) 1 0.60 1.5  0.00 2 0.60 1.5 0.5 3 0.60 1.5 1.0 40.60 1.5 2.0 5 0.60 1.5 3.0

[0081] TABLE 4 Weight of the individual constituents for the mortarspecimens. Specimen Cement (lb.) Water (lb.) Sand (lb.) Salt (lb.) 123.444 14.066 29.472 0000 2 23.444 14.066 29.472 0.117 3 23.444 14.06629.472 0.234 4 23.444 14.066 29.472 0.469 5 23.444 14.066 29.472 0.703

[0082] TABLE 5 Compressive strengths measured with the cylinders for w/c= 0.60. NaCl/c (%) Height (in.) Strength (psi) 0.00 7⅝ 4521.2 0.00 7½4418.8 0.00 7⅝ 5256.7 0.00 7{fraction (11/16)} 4439.7 0.10 7⅝ 5198.60.10 7{fraction (11/16)} 5945.9 0.10 7{fraction (13/16)} 5235.9 0.10 7¾5398.9 0.20 7⅝ 6141.6 0.20 7{fraction (11/16)} 5750.4 0.20 7{fraction(13/16)} 5810.9 0.20 7⅝ 5918.0 0.30 7{fraction (11/16)} 4579.4 0.307{fraction (11/16)} 4328.0 0.30 7{fraction (11/16)} 4486.3 0.30 7⅝4553.8 1.0  7¾ 5440.8 1.0  7¾ 4889.0 1.0  7¾ 5038.0 1.0  7⅞ 4619.0 2.0 7¾ 5557.1 2.0  7¾ 4630.6 2.0  7⅝ 5606.1 2.0  7{fraction (11/16)} 5212.63.0  7¾ 5913.4 3.0  7{fraction (11/16)} 6064.7 3.0  7{fraction (6/16)}6445.8 3.0  6{fraction (15/16)} 5665.0

[0083] TABLE 6 Compressive strengths measured with the cylinders for w/c= 0.60. NaCl/c Strength (psi) Average Strength 0.00 2387.3 2407.2 0.002387.3 0.00 2307.7 0.00 2546.5 0.50 4138.0 4085.5 0.50 4138.0 0.503978.9 0.50 3978.9 1.0  3342.3 3302.5 1.0  3342.3 1.0  3342.3 1.0 3183.1 2.0  3382.0 3561.1 2.0  3740.1 2.0  3382.0 2.0  3740.1 3.0 4098.2 4118.1 3.0  4217.6 3.0  4138.0 3.0  4018.7

[0084] TABLE 7 Conversion from NaCl/c percentage to actual Salinitypercentage. NaCl/c (%) 0.5 Salinity (%) 0.6 Salinity (%) 0.1 0.2 N/A 0.20.4 N/A 0.3 0.6 N/A 0.5 N/A 0.832 1.0 2.0 1.664 Sea water ˜3.25 ˜3.252.0 4.0 3.334 3.0 6.0 4.998

[0085]FIGS. 3 through 10 show the results of 0.50 w/c ratio with varyingNACI/c ratio. As expected, there is an exponential decrease in themagnitude of reflection coefficient, |Γ|, as a function of curing time.FIGS. 11 through 12 show the results of 0.60 w/c ratio with varyingNaCl/c ratio. Again, a similar trend of |Γ| as a function of time isobserved. The exponential decrease is due in part to the evaporation ofthe free water molecules during the curing period and the chemicalbonding of water to the cement molecules. Since microwave is highlysensitive to the presence of water, especially free water, themeasurement of |Γ| is expected to decrease as a function of curing time.Comparing both S-band plots (FIGW. 3 and 11) of 0.50 and 0.60 w/cratios, the measurement of |Γ| for the specimens with w/c=0.50 isconsistently greater than those containing 0.60 w/c ratio. Additionally,per given w/c ratio, the data shows distinction between percentage ofsodium chloride for each specimen.

[0086]FIGS. 7 and 8 show the measurements of |Γ| at 10 GHz (X-band) forcertain specimens. At the end of the 28 day curing period, themeasurements of r are greater for 0.50 w/c ratios than for 0.60 w/cratios. This is due to the fact that for any mortar specimen there is afinite amount of cement that the water can bind to. Any free water willsimply evaporate. In specimens with 0.50 w/c ratio, there is more cementfrom which the water can bind to and therefore this specimen willcontain more bound water. Microwaves are still sensitive to bound water.Therefore, the measurement of |Γ| is expected to be greater forspecimens with lower w/c ratio. For the present problem of chloridedetection and content determination, we see that the data shown withX-band frequencies is not very conclusive, since the difference in themeasurement of |Γ| as a function of chloride content is not practicallymeasurable. At 10 GHz there is less distinction between the permittivityfor varying NaCl/c. Since the measurement of |Γ| is proportional to thedielectric property of the specimen, less difference is expected in themeasurement in this frequency range. Ideally, the measurements should beperformed at lower frequencies (i.e., S-band). FIGS. 3 and 5 prove thereliability of |Γ| as a function of chloride content. At day 28, thedistinction between |Γ| distinguishable as compared to the 10 GHz plots.Given that these points are distinguishable, data is prepared using theS-band frequencies.

[0087]FIGS. 13 and 14 show the compressive strength of the cylindersafter the 28-day curing period. Here the compressive strength ofspecimens with 0.50 w/c ratio is expected to be higher than that with0.60 w/c ratio. This behavior would conform to Abrahm's Law. Thesefigures show that w/c 0.60 has less strength than the w/c =0.50.Additionally, per given w/c ratio, the compressive strength increaseslinearly as a function of NaCl/c ratio. Therefore one could be lead tobelieve that from a compressive strength standpoint, the addition ofchloride in cement based materials is beneficial. However, the downsideto this stream of thought is that the protective film mentioned earlierhas a greater risk of breaking down and deteriorating the steel members.This would compromise the structural integrity of the structure (reducelateral load resistance) and possibly degrade its compressive strength.

[0088] In FIGS. 15 and 16, the influence of chloride content on curingrate (θ|Γ| θt) is addressed. The addition of chloride in cement basedmaterials will accelerate its initial setting time (i.e., hardening) butnot change its final curing time. Therefore a greater percent of theinitial curing process would occur in the first day of curing for thechloride contaminated specimens. This would translate into less changein |Γ| as a function of days for the remainder of the 28-day curingperiod, and a smaller reading of θ|Γ| θt. From FIGS. 15 and 16, we seethat the specimen with 0.0% NaCl/c (non-contaminated specimen) takesmore time to set, and therefore the measurement of a θ|Γ| θat is greaterin the first days of curing. However, the total curing time is notreduced by the present chloride, since all mixtures converge to theirfinal value of |Γ| after approximately 16-17 days (i.e., 0).

[0089] Lastly, since the effect of NaCl/c on the measurement of |Γ| andcompressive strength has been discussed, it would be interesting todetermine if one could determine a relationship of |Γ| vs. compressivestrength. In FIGS. 17 and 18, there is a linear relationship betweenNaCl/c and measurements of |Γ|. In FIGS. 19 and 20, it is shown thatcompressive strength increases linearly with respect to NaCl/c ratio.Therefore the relationship between |Γ| and compressive strength shouldalso be linear. This behavior is readily observed in FIGS. 19 and 20.Therefore the nondestructive determination of compressive strength ofmortar as a function NaCL/c ratio could be possible by using a linearinterpolation scheme with respect to an uncontaminated specimen. FIG. 21indicates that the relative permittivity of water for differentsalinities decreases as a function of frequency. FIG. 22 illustratesthat the loss factor of water increases with greater salinity and thedifferences therebetween can be more readily determined at relativelylower frequencies.

[0090] With reference to FIG. 23, an electromagnetic theoretical modelfor calculating the reflection properties of a multi-layered dielectriccomposite structure inspected by an open-ended rectangular waveguide hasbeen developed. The input parameters to this model include the frequencyof operation, the number of layers, the thickness of each layer (t) andthe dielectric properties (ε_(r)) of each layer.

[0091] Consider each specimen to be modeled as a multi-layered materialafter successive exposure to the chloride solution. As mentionedearlier, it is intended to use relatively low microwave frequencies forall specimens including concrete. Consequently, all specimens can beconsidered to be homogeneous at these frequencies, and the influence ofdiscrete aggregates in concrete will be at best minimal and can beneglected. Hence, the homogeneous layered model of a concrete specimenis valid for this modeling effort. For each specimen, its dielectricproperties will be calculated from its measured reflection coefficientbefore it is exposed to chloride and after exposure to successiveaccelerated chloride contamination. This information is then used as aclose estimate of the dielectric properties of each layer of thediscretely modeled specimen. From the actual measurements of some of thedrilled cores, we will obtain information about the manner by which thechloride has penetrated into each specimen. From this information acertain trend for chloride penetration into the specimen can be formed;namely, a linear or exponential chloride penetration trend.Additionally, the depth to which chloride may have penetrated will alsobe known. Subsequently, using these pieces of information we candiscretize the depth of penetration to estimate the number of layers forthe model and their respective dielectric properties. Once thereflection coefficient of a specimen is calculated using this model, itwill be compared to the measured reflection coefficient. The dielectricproperty trend (indicator of the trend of chloride penetration) and thenumber of layers will then be iteratively modified to get a closeragreement between the measured and the calculated reflectioncoefficients. The results of this model can then be used as a predictorof chloride content and the extent (depth) of its penetration forunknown concrete specimens in practical environments. This is to saythat once the reflection coefficient of an unknown concrete specimen ismeasured the chloride level and its depth ofpenetration may bedetermined using the results of this model.

[0092] With reference to FIG. 24, the depth to which a microwave signalpenetrates inside a dielectric material, such as the specimens ofinterest in this proposal, is a function of the dielectric properties ofthe material, the frequency of operation and the incident power level.In addition, this parameter is also influenced by the sensitivity of theapparatus (including a HP8510 network analyzer in this case) since amore sensitive detector allows reflected signals from deeper portions ofa material to be detected. For the frequencies of interest and theconcrete based materials for investigation, using the HP8510 networkanalyzer, the depth from which the reflected signal may be detected isseveral centimeters (7-10 cm). The dashed line in FIG. 24 qualitativelyshows the approximate relative depth from which the microwave signals inprevious investigations have been detected.

[0093] This depth of microwave signal penetration also closely followsthe depth at which reinforcing bars are located in practical reinforcedconcrete. If deeper interrogation of a concrete specimen is required,then one may increase the incident microwave power level.

[0094] Microwave NDT determination of content and curing effect ofchloride in cement based materials was successfully demonstrated. Thework done in this area can be effective in using the near-fieldtechniques discussed above. The research done to this point has strongimplications in that chloride detection is feasible. The data shows thatby using S-band frequencies and analyzing the magnitude of reflectioncoefficient, detection between uncontaminated and contaminated specimenscan be accomplished. The information provided can relate the linearincrease in magnitude of reflection coefficient to the increase incompressive strength. This connects the electrical properties to themechanical properties. Although, adding chloride to the cement materialsappears to increase the compressive strength, there is also greater riskof deteriorating the steel members. This would compromise the structuralintegrity of the structure and possibly degrade its compressivestrength.

[0095] More information is next provided including FIGS. 25-34 relatedto penetration of salt or chloride material in cement or mortarspecimens. From this, chloride model information can be ascertainedrelated to establishing a model, such as in the form of a graph, anequation, and/or data in a look-up table, related to correlatingdielectric property information (e.g., related to the magnitude of thereflection coefficient) and chloride content information in the cementspecimen. Among the steps conducted and relevant background informationare the following:

[0096] Mortar specimens are prepared with varying w/c and s/c. The curedspecimens are placed in a salt bath for a known time and then removed(FIG. 2). The reflection properties of these specimens are measuredusing open-ended rectangular waveguides at microwave frequencies (FIG.1).

[0097] The specimens are cored and their dielectric properties aremeasured as a function of depth into these specimens. A multi-layerelectromagnetic code is used to predict the amount and depth of chloridepenetration. A determination is made whether such models can beeventually used for predicting chloride content and depth to which itmay have penetrated.

[0098] Chloride ions can be introduced in cement-based structures indifferent ways: (a) during its manufacture: (i) mixing water and (ii) inthe sand and aggregate; (b) after the construction of a structure: (i)de-icing salts and (ii) exposure to salt air.

[0099] Various amounts of table salt were added to the mixing water ofvarious cement paste and mortar specimens. The specimens weremoist-cured for one day and then cured at room temperature and humidityfor the remainder of the prescribed 28-dayperiod. Their reflectionproperties were measured daily at S- and X-band.

[0100] The magnitude of reflection coefficient, |Γ|, showed to be auseful parameter for detecting and evaluating the presence of salt. |Γ|,at 3 GHz (S-band), was correlated to the salt content and thecompressive strength of these specimens. This correlation was shown tobe systematic as a function of w/c in mortar.

[0101] A mortar specimen can be placed into salt water for a certainamount of time (originally under pressure). Calcium chloride was used aswell sodium chloride. The specimen was dried in the ambient environmentfor approximately 24 hours. The reflection -15 coefficient of thespecimen was measured daily until no change was sensed.

[0102] This process was repeated for a few cycles. The specimen wascored, cut and ground.

[0103] Their dielectric properties were measured. Core samples wereanalyzed using electron microscopy and X-ray fluorescence. Such data wasattempted to be correlated with the chloride ingress.

[0104] A monopole antenna probe may be useful in connection with makingmeasurements to determine information related to materials found infresh cement paste and concrete, such as the water/cement (w/c) ratioand/or chloride content. A monopole probe is similar in shape and sizeto a pin with a needle-like probe extending from its end, which can beinserted, for example, into fresh cement paste. The microwave propertiesof a monopole probe are controlled by it length, frequency of operation,the coaxial line geometry used to excite it and the dielectricproperties of the medium surrounding it (e.g., fresh Portlandcement-based material). As this medium changes from free space to freshcement paste, as this probe is inserted into a fresh batch of concrete,so do the measured microwave reflection properties of the monopole. Thisinformation can be obtained in real-time and it is anticipated that suchcan be correlated to chloride content and/or the w/c ratio. When theprobe is inserted to a fresh batch of concrete, the influence ofaggregates can be reduced or eliminated by the choice of the operatingfrequency, monopole size and operating microwave power.

[0105] A typical monopole probe is an extension of an inner conductor ofa coaxial transmission line whose outer conductor is commonly terminatedin an infinite ground plane (in theory). In practice and depending onthe dielectric properties of the medium under inspection, the extent ofthis ground plane may be relatively small or it may be altogethereliminated. Based on the dielectric properties of the medium, such asfresh cement paste, the probe design can be optimized for determiningoptimal dimensions thereof, as a function of frequency. The optimalprobe dimensions (e.g., length and diameter) are those which give highersensitivity to the measurement being made, such as related to chloridecontent and/or w/c ratio.

[0106] The foregoing discussion of the invention has been presented forpurposes of illustration and description. Further, the description isnot intended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with the aboveteachings, within the skill and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedhereinabove are further intended to explain the best modes presentlyknown of practicing the invention and to enable others skilled in theart to utilize the invention in such, or other embodiments, and with thevarious modifications required by their particular application or usesof the invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

What is claimed is:
 1. A method for determining chloride content in asample including cement, comprising: providing chloride modelinformation; sending a transmit signal having a predetermined frequencyto the cement sample; receiving a return signal from the cement sample;and determining chloride content information in the cement sample usingat least said return signal and said chloride model information.
 2. Amethod, as claimed in claim 1, wherein: said chloride model informationis stored in memory in a form that includes at least one of thefollowing: an equation, a graph and a look-up table related tocorrelating dielectric property information and chloride content incement.
 3. A method, as claimed in claim 1, wherein: said chloride modelinformation is developed before conducting said sending and receivingsteps and said chloride model information was developed using thefollowing steps: providing a cured cement specimen; allowing forpenetration of chloride material into the cured cement specimen;obtaining data related to dielectric property information associatedwith the cured cement specimen; and utilizing said data to provide saidchloride model information related to correlating said dielectricproperty information and said chloride material.
 4. A method, as claimedin claim 3, wherein: said allowing step includes placing the curedcement specimen in a salt bath for a known time interval.
 5. A method,as claimed in claim 3, wherein: said obtaining step includes measuringthe magnitude of a reflection coefficient that has said dielectricproperty information.
 6. A method, as claimed in claim 5, wherein: saidobtaining step includes analyzing at least a first portion of the curedcement specimen removed from other portions of the cured cement specimenafter said allowing step.
 7. A method, as claimed in claim 6, wherein:said measuring step is conducted using a first instrument and saidanalyzing step is conducted using a second instrument and in which saidsecond instrument includes at least one of electron microscopy and X-rayfluorescence.
 8. A method, as claimed in claim 1, wherein: said chloridecontent information relates to at least one of an amount of the chlorideand a depth to which the chloride has penetrated.
 9. A method, asclaimed in claim 1, wherein: said chloride model information wasgenerated before said sending and receiving steps and said chloridemodel information was generated using the following steps: providing anumber of cement specimens including at least a first cement specimenand a second cement specimen; placing the first cement specimen in asalt bath for a known time interval; removing the first cement specimenfrom the salt bath; permitting the first cement specimen to dry;measuring a magnitude of a reflection coefficient of the first cementspecimen; and continuing measuring said reflection coefficient of thefirst cement specimen at periodic intervals until any change in saidmagnitudes of reflection coefficients are substantially the same or arewithin an acceptable variation of each other.
 10. A method, as claimedin claim 3, wherein: said obtaining step includes coring the cementspecimen and analyzing at least a first portion of said cored cementspecimen.
 11. A method, as claimed in claim 11, further including:cutting and grounding said first portion of said cored cement specimen.12. A method, as claimed in claim 9, wherein: said measuring step isconducted using an instrument substantially equivalent to an instrumentused in conducting said determining step.
 13. A method involving apredetermined material that may be in a sample which includes cement,comprising: providing model information; sending a transmit signalhaving a desired frequency to the cement sample; receiving a returnsignal from the cement sample; and determining information related tosaid predetermined material in the cement sample using at least saidreturn signal and said model information; wherein said model informationis generated before conducting said sending and receiving steps using atleast the following: allowing penetration of predetermined material intoat least a first cement specimen after the first cement specimen hasbeen cured and obtaining data related to dielectric propertycharacteristics associated with the cured first cement specimen.
 14. Amethod, as claimed in claim 13, wherein: said allowing step includesplacing the cured first cement specimen in a bath that includespredetermined material.
 15. A method, as claimed in claim 13, wherein:said model information is obtained from a plurality of cured cementspecimens including a cured first cement specimen and a cured secondcement specimen and in which the cured first cement specimen issubjected to said predetermined material for a first time period and thecured second specimen is subject to said predetermined material for asecond time period.
 16. A method, as claimed in claim 13, wherein: saidmodel information includes chloride model information and said allowingstep includes placing the cured first cement specimen in a saltcontaining bath.
 17. A method, as claimed in claim 13, wherein: saidmodel information is obtained using data resulting from analyzing aremoved portion from said cured cement specimen using at least one ofelectron microscopy and x-ray fluorescents.
 18. An apparatus for makingdeterminations related to a predetermined material in a cement sample,comprising: a signal generating subsystem for outputting microwavesignals; a coupler subsystem for transmitting microwave signals receivedfrom said signal generating subsystem and for receiving return signalsfrom the cement sample; and an analyzing subsystem in communication withsaid coupler subsystem that determines information related to thepresence of said predetermined material in the cement sample, saidanalyzing subsystem including at least one memory that stores modelinformation related to correlating dielectric property information andcontent of said predetermined material in cement, said model informationbeing based on a number of cured cement specimens including cured firstand second cement specimens, with the cured first and second cementspecimens having been subject to predetermined material for first andsecond time periods, respectively.
 19. An apparatus, as claimed in claim19, wherein: the amount of said predetermined material in the curedfirst cement specimen after being subjected to said predeterminedmaterial is greater than the amount of said predetermined material inthe cured first cement specimen before being subjected to saidpredetermined material.
 20. An apparatus, as claimed in claim 18,wherein: said model information includes chloride model information.